High selectivity etch using an external plasma discharge

ABSTRACT

An apparatus and method for scavenging etchant species from a plasma formed of etchant gas prior to the etchant gas entering a primary processing chamber of a plasma reactor. There is at least one scavenging chamber, each of which is connected at an inlet thereof to an etchant gas source and at an outlet thereof to a gas distribution device of the primary processing chamber. Each scavenging chamber has a radiation applicator that irradiates the interior of the scavenging chamber and creates a plasma therein from etchant gas flowing through the chamber from the etchant gas source to the gas distribution apparatus of the primary processing chamber. The applicator uses either an inductive discharge, capacitive discharge, direct current (DC) discharge or microwave discharge to irradiate the interior of the scavenging chamber and ignite the plasma. An etchant species scavenging source is also disposed within the scavenging chamber. This source provides scavenging material that interacts with the plasma to scavenge etchant species created by the dissociation of the etchant gas in the plasma and form etch by-products comprised of substances from both the etchant species and the scavenging source. The scavenging chambers can be employed, as is or in a modified form, as excitation chambers to excite gases at optimal conditions and feed the modified gases into the primary chamber. The scavenging chamber is modified by removing its scavenging source if this source would adversely interact with the gas being excited.

This application is a divisional of application Ser. No. 09/020,959,filed on Feb. 9, 1998, now U. S. Pat. 6,074,514.

BACKGROUND

1. Technical Field

The invention is related to a plasma reactor for processing a workpiecesuch as a semiconductor wafer or insulating substrate wherein etchselectivity is enhanced by scavenging etchant species from the plasma,and more particularly to such a reactor wherein the scavenging processis conducted outside the processing chamber of the reactor.

2. Background Art

A plasma reactor may be employed to perform various processes on asemiconductor wafer in microelectronic fabrication. The wafer is placedinside a vacuum chamber of the reactor and process gases, includingetchant gases, are introduced into the chamber. The gases are irradiatedwith electromagnetic energy to ignite and maintain a plasma. Dependingupon the composition of the gases from which the plasma is formed, theplasma may be employed to etch a particular material from the wafer ormay be employed to deposit a thin film layer of material onto the wafer.

An important factor associated with using a plasma reactor for etchingis the etch selectivity. The term etch selectivity refers to the ratioof etch rates of two different materials on a workpiece undergoingetching in the plasma reactor. In one common scenario, it is desiredthat oxygen-containing materials on a workpiece be etched much fasterthan an overlying mask formed of photoresist or so-called hardmaskmaterial (e.g. SiO₂ or Si₃N₄). Additionally, it is often desired thatthe oxygen-containing materials be etched much faster thannon-oxygen-containing materials of the workpiece. These comparative etchrate relationships are referred to as a high oxide-to-mask andoxide-to-“nonoxide” selectivity, respectively. The desirability of thishigh selectivity will be explained using the example of etching acontact opening through a dielectric layer, such as silicon dioxide(SiO₂), to an underlying polysilicon conductor layer and/or to a siliconsubstrate of a semiconductor wafer. A layer of mask material is formedover the surface of the silicon dioxide layer prior to the etchingprocess in those areas that are not to be etched. Accordingly, there isno mask formed in the area where the contact opening is to be etched.The desired result of the etching process is to quickly etch through thesilicon dioxide layer where the contact opening is to be formed, but notto significantly etch the surrounding mask, or the polysilicon orsilicon material (or other non-oxygen-containing material such assilicon nitride) underlying the silicon dioxide layer. Thus, highoxide-to-mask and oxide-to-silicon etch selectivities are desired. For asilicon oxide etch process, process gases including an etchant such asfluorine-containing gases are introduced into the chamber. Thefluorine-containing gases freely dissociate under typical plasmaconditions so much that not only is the silicon oxide layer etched butthe mask and the eventually exposed underlying polysilicon or siliconmaterials are also etched to an unacceptable degree. Thus, withouttaking steps to ameliorate the effect of excess fluorine-containingetchant species in the plasma on the mask and non-oxide layers of thewafer, a less than desirable etch selectivity results. In fact, if theselectivity is low enough a so-called “punch through” condition canresult wherein the mask layer or a non-oxide layer is etched throughcausing damage to the device being formed on the wafer. Similar problemsrelated to excess etchant species in the plasma occur in other etchprocesses as well. For example, polysilicon and silicide (gate) etchprocesses, or metal etch processes, are subject to degraded selectivityin the presence of excess etchant species.

One method of dealing with the excess of etchant species in the plasmais to introduce a substance that combines with some of the etchantspecies to form non-etching substances. This process is typicallyreferred to as “scavenging”. Ideally, just enough of the etchant speciesis scavenged from the plasma to increase the selectivity withoutreducing the etching rate of the material being etched to anunacceptable degree. For example, in the previously-described silicondioxide etch process, fluorine etchant species are scavenged from theplasma typically by introducing silicon to form the non-etchingby-product SiF₄. This silicon can be introduced as a component of a gas,or via a solid silicon-containing structure such as one containing puresilicon, polysilicon, silicon carbide (SiC), or a silicon-baseddielectric. In the case where a solid silicon-containing source isemployed, the source can form a part of the reactor chamber ceilingand/or walls, or it can be a separate piece held within the chamber.Typically, the temperature of the solid silicon-containing source iscontrolled to prevent it from being covered with deposits comprised ofetch by-products or a polymer film (as will be more fully discussedlater), and additionally to permit silicon to be more easily removedfrom the source by the plasma in desired quantities. An RF biaspotential is also often applied to a solid silicon-containing source inconjunction with controlling the temperature for the same reasons.

However, in some etching processes the selectivity cannot be increasedto satisfactory levels without unacceptably reducing the etch rate ofthe material being etched from the workpiece. In these situations it isknown to introduce a substance into the plasma which causes aprotective, etch-resistant layer to deposit on the workpiece materialsthat are not to be etched, while not depositing on the material to beetch to any significant degree. For example, in the aforementionedsilicon dioxide etch process, it is known that the oxide-to-mask andoxide-to-silicon etch selectivity is enhanced by a polymer film thatforms more readily over the mask, silicon, polysilicon, and othernon-oxygen-containing layers than over silicon dioxide (or otheroxygen-containing materials). The polymer resists etching by thefluorine etchant species, thereby increasing the aforementionedselectivity. One common method of forming such a selectivity-enhancingpolymer film is to employ a fluorocarbon or fluoro-hydrocarbon gas(e.g., ethyl hexafluoride (C₂F₆) or trifluoromethane (CHF₃)) as thefluorine-containing portion of the process gas. Some of thefluorine-containing species in the plasma are consumed in etching thesilicon dioxide layer on the wafer. Other species form a polymer layeron the surface of the wafer. This polymer forms more rapidly andstrongly on any exposed non-oxygen-containing surface, such as the mask,silicon or polysilicon surfaces, than on the oxygen-containing surfacessuch as the silicon dioxide. In this way the non-oxygen-containingsurfaces are protected from the action of the fluorine etching speciesand the etch selectivity for those surfaces is enhanced. The etchresistance of the polymer can be further strengthened by increasing theproportion of carbon in the polymer relative to fluorine. Typically, thepreviously-described fluorine scavenging process is employed to reducethe amount of free fluorine in the plasma, thereby resulting in anincrease in the carbon content of the polymer.

It is evident from the foregoing description that the scavenging processplays key role in producing a desired etch selectivity in mostplasma-enhanced etching processes, including those relying on theformation of a protective film such as the carbon-fluorine polymeremployed in silicon oxide etch procedures. However, current etchantspecies scavenging processes have drawbacks. For example, in the case ofa solid silicon scavenging source, one problem is that the rate ofremoval of silicon from the source required to achieve the necessarydecrease in the free fluorine etchant species population of the plasmais so great that the source is rapidly consumed and the consequent needto idle the plasma reactor to replace the source exacts a price in lossof productivity and increase costs. In addition, the size of a modernplasma reactor chamber dictates that the solid silicon source, whetherit be integrated into the ceiling and/or walls of the chamber, or aseparate piece supported within the chamber, be relatively large so thatthe scavenging process is uniform across the width of the plasma. Thispresents a problem as it is difficult to manufacture and control thepurity of large silicon structures. As a result, these structures areexpensive. Further, this problem is likely to become even worse in viewof the current trend to increase the size of the reactor chamber toaccommodate ever larger workpieces. The larger reactor chambers willrequire even bigger silicon sources with a corresponding increase inprice.

Another problem with current scavenging processes concerns the devicesrequired to control the temperature of a solid scavenging materialsource. Typically, the temperature control devices are integrated withthe source to reduce the time it takes to change its temperature,thereby ensuring the temperature can be carefully controlled throughoutthe etch process. This need to integrate portions of the temperaturecontrol device into the source itself complicates the structure further,thereby making it even more difficult to manufacture and more expensive.In addition, the larger the source, the more elaborate the temperaturecontrol device has to be in order to ensure a precise control of thesource's temperature. Given the aforementioned trend toward up-sizingthe reactor chambers, the cost of these consumable scavenging sourcesmay become exorbitant.

An even greater problem with current scavenging processes is that theprocess parameters, such as the RF power level or chamber temperature,which lead to optimizing etching of the workpiece are not typicallythose that will maximize selectivity. For example, it is known thatincreasing the RF power input into the chamber can boost the etch rate.However, this same increase in power also tends to increase theconcentration of free etchant species in the plasma which can lead to anundesirable lowering of the oxide-to-mask or oxide-to-nonoxideselectivity. Thus, there is an troublesome tradeoff between the etchrate and selectivity.

Accordingly, there is a need for a plasma reactor design and method ofscavenging etchant species from the plasma that does not require the useof large, expensive, scavenging source structures within the reactor'sprocessing chamber which require costly and frequent replacement.Further, there is a need for such a reactor design and scavenging methodthat decouples the control of etch selectivity from the control of etchperformance, thereby eliminating the undesirable tradeoff between theseetch process factors.

SUMMARY

The stated needs are fulfilled by an apparatus and method for scavengingetchant species from a plasma formed of etchant gas in a separatescavenging chamber prior to the etchant gas being introduced into theprimary processing chamber of the reactor. Scavenging etchant speciesfrom the etchant gas prior to feeding it into the primary chamber“loads” the gas with relatively stable, non-etching, etch by-productsformed in part from what would have otherwise become etchant species inthe plasma created within the primary processing chamber and in partfrom the material used as a scavenging source in the scavenging chamber.In this way, the concentration of etchant species in the plasma of theprimary processing chamber is reduced and the concentration of etchby-products in the plasma is increased, ideally to levels that maximizethe oxide-to-mask and oxide-to-nonoxide etch selectivity of the etchprocess being performed in the reactor. The net result of this method ofscavenging etchant species is to eliminate the need for any type ofscavenging source structure inside the primary processing chamber of thereactor. Thus, the expense of these large primary chamber scavengingsource structures is avoided, as is the cost associated with opening theprimary chamber to replace the source.

The preferred apparatus is a plasma reactor that in addition to itsprimary processing chamber includes at least one scavenging chamber.Each scavenging chamber is connected at an inlet thereof to an etchantgas source and at an outlet thereof to a gas distribution device of theprimary processing chamber. Each scavenging chamber has a radiationapplicator capable of irradiating the interior of the scavenging chamberand creating a plasma from etchant gas flowing therethrough from theetchant gas source to the gas distribution apparatus of the primaryprocessing chamber. The applicator can be of the type that uses eitheran inductive discharge, capacitive discharge, or microwave discharge toirradiate the interior of the scavenging chamber and ignite the plasma.An etchant species scavenging source is also included within thescavenging chamber. This source is capable of providing scavengingmaterial that interacts with the plasma to scavenge etchant speciescreated by the dissociation of the etchant gas in the plasma to formnon-etching by-products comprised of substances from both the etchantspecies and the scavenging source.

The etchant species scavenging source is made of a material that willmodify the etchant gas in a way that decreases the etch rate of a targetmaterial of a workpiece undergoing etch processing in the primaryprocessing chamber of the plasma reactor, thereby increasing theselectivity for the target material. For example, the scavenging sourcematerial would preferably be of a type that scavenges the kind ofetchant species from the plasma formed within the scavenging chamber andproduces the kind of etch by-products that results in the aforementionedlowering of the etch rate of the target material during processing. Ifpractical, the scavenging source could be made of the target materialitself to obtain the desired results.

The etchant species scavenging source is also preferably solid and atleast partially made of a solid scavenging material. If so, it is alsopreferred that the scavenging chamber have a removable lid covering anaccess opening in the chamber. The scavenging source is sized so as tofacilitate its being installed into or removed from the chamber throughthe access opening. Additionally, if the source material is of a typethat is expensive and difficult to form into relatively largestructures, it is preferred that the source be as small as possiblewhile still being capable of scavenging sufficient etchant species tocreate a desired concentration of etchant species and etchantby-products in the plasma formed within the primary processing chamber.The power applied to the radiation applicator can be increased and/orthe flow rate of the etchant gas through the scavenging chamber can bedecreased in order to increase the scavenging capability of thescavenging source, thereby allowing a smaller source to be used in thechamber. A solid source can also incorporate a temperature controlapparatus if desired. The temperature control apparatus is capable ofcontrolling the temperature of the scavenging source. Controlling thesource temperature provides yet another way of ensuring the scavengingof sufficient etchant species to create a desired concentration ofetchant species and etchant by-products in the plasma.

The amount of scavenging, and so the selectivity exhibited in theprimary processing chamber, can further be actively controlled via theaforementioned methods. Namely, the power input to the scavengingchamber via the radiation applicator can be adjusted to control theamount of scavenging in the etchant gas flowing through the scavengingchamber. Likewise, the flow rate of the etchant gas through thescavenging chamber can be adjusted to control the scavenging. Andfinally, the temperature of the scavenging source can be adjusted (if atemperature control apparatus is incorporated) to control the amount ofscavenging. It is noted that the above-described control methods allowthe selectivity exhibited during etch processing in the primary chamberto be for the most part independently determined regardless of theprocessing parameters employed in the primary chamber to optimize etchperformance. The selectivity-determining scavenging has already occurredbefore the etchant gas even reaches the primary chamber. Thus, thecontrol of selectivity has truly been decoupled from the control of etchperformance using the methods of the present invention, and so theaforementioned tradeoff eliminated.

Alternately, the etchant species scavenging source can be a gaseousscavenging material introduced into the scavenging chamber. Preferably,the gaseous scavenging material would be introduced in a sufficientquantity to create a desired concentration of etchant species andetchant by-products in a plasma formed within the primary processingchamber.

As stated previously, there is at least one scavenging chamber connectedto the primary processing chamber of the plasma reactor in accordancewith the present invention. However, employing. multiple scavengingchamber can be particularly advantageous. For example, it is oftendesired to increase the selectivity to more than one material on theworkpiece. These separate materials may require that a different etchantspecies to be scavenged and/or different etching by-products to beloaded into the etchant gas in order to optimize the selectivity. If so,employing multiple scavenging chambers will allow for a separatetailoring of the selectivity of the materials. Essentially, thescavenging material of the source disposed in each scavenging chamber ischosen to scavenge the type of etchant species and produce the type ofetch by-product necessary to create the desired selectivity for aparticular material of interest on the workpiece. If two such materialsare of interest, two scavenging chambers are employed. If it is desiredto increase the selectivity for three such materials on the workpiece,three scavenging chambers are employed, and so on.

Further, the above-described scavenging chambers can be employed, as isor in a modified form, as excitation chambers to solve another problemtypical of etch processing in a plasma reactor. The process parameterspromoting optimal etch conditions (such as a fast etch rate) within theprimary processing chamber are also not necessarily conducive tooptimizing certain other process factors, exclusive of selectivity. Forexample, inert gases such as argon (Ar) and helium (He) are oftenexcited in the plasma of the primary processing chamber of a plasmareactor for advantageous effect. However, the process parameters thatpromote optimal etch performance in the primary chamber, such as the RFpower level, are not always conducive to effectively excite these inertgas components of the processing gas. The problem can be resolved byemploying a separate excitation chamber outside of the primaryprocessing chamber that excites gases at optimal conditions and feedsthe modified gases into the primary chamber. The previously-describedscavenging chamber can readily act as such an excitation chamber, theonly caveat being that the scavenging source may adversely interact withthe gas being excited in some cases. However, as the source is removablefrom the scavenging chamber, it can simply be removed if such an adverseinteraction would occur. Alternatively, one or more dedicated excitationchambers, identical to the previously described scavenging chamber butnot capable of supporting a scavenging source, could be incorporatedinto the reactor, if desired.

In addition to the just described benefits, other objectives andadvantages of the present invention will become apparent from thedetailed description which follows hereinafter when taken in conjunctionwith the drawing figures which accompany it.

DESCRIPTION OF THE DRAWINGS

The specific features, aspects, and advantages of the present inventionwill become better understood with regard to the following description,appended claims, and accompanying drawings where:

FIG. 1 is a schematic diagram of a plasma reactor incorporating aseparate scavenging chamber in accordance with the present invention.

FIG. 2A is a cross-sectional, side view of the scavenging chamber ofFIG. 1 employing an inductive power applicator and an internalscavenging liner.

FIG. 2B is a partially cut-away, side view of the scavenging chamber ofFIG. 2A wherein the internal scavenging liner is made of a conductivematerial and slotted to admit RF power into the interior of the chamber.

FIG. 3A is a cross-sectional, side view of the scavenging chamber ofFIG. 1 employing a capacitive power applicator comprising a pair ofoppositely facing internal electrodes made of a scavenging material.

FIG. 3B is a partially cut-away, side view of the scavenging chamber ofFIG. 1 employing a capacitive power applicator comprising a pair ofoppositely facing external electrodes and an internal scavenging liner.

FIG. 4 is a partially cut-away, side view of the scavenging chamber ofFIG. 1 employing a microwave source power applicator and a scavengingliner chamber.

FIG. 5 is a schematic diagram of a plasma reactor incorporatingmultiple, separate scavenging chambers in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the preferred embodiments of the presentinvention, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. It is understoodthat other embodiments may be utilized and structural changes may bemade without departing from the scope of the present invention.

The proposed scavenging of etchant species from the etchant gas prior toit reaching the primary processing chamber 102 of the reactor 100according to the present invention is accomplished by incorporating ascavenging chamber 104 into the reactor, as depicted in FIG. 1. Thisscavenging chamber 104 is located outside the primary processing chamber102 of the reactor. The etchant or reactive gas which would have been apart of the processing gas heretofore fed directly into the primaryprocessing chamber 102 of the reactor is instead first fed through thescavenging chamber 104. In the embodiment of the present inventiondepicted here, the remaining constituents of the processing gas are fedinto the primary processing chamber 102 in the normal manner. A plasmais formed within the scavenging chamber 104 from the etchant gas as itflows through this chamber. The etchant gas disassociates within theplasma to form, among other things, etchant gas species. A scavengingmaterial source (not shown) included in the scavenging chamber 104 ispositioned so as to react with the plasma. The interaction of the plasmaand the scavenging material source reduces the concentration of freeetchant gas species to the desired level and creates by-products havingcomponents from both the etchant species and the source. The nowmodified etchant gas flows out of the scavenging chamber 104 and intothe primary processing chamber 102 where it mixes with the otherconstituents of the processing gas. This mixing can occur within theprimary chamber itself or preferably in the manifold of a conventionalgas distribution apparatus 106 capable of feeding the combined gasesinto the primary chamber.

A prime advantage of performing etchant species scavenging outside ofthe primary processing chamber is to allow the control of etchselectivity to be decoupled from the control of etch performance. Asdiscussed previously, current scavenging processes employing a solidscavenging material source resident within the primary processingchamber are detrimentally affected by attempts to optimize etchperformance. Setting certain process parameters, such as the RF powerlevel or chamber temperature, to levels that benefit etchingperformance, such as by increasing the etch rate, can simultaneouslyincrease the concentration of free etchant species in the plasma. Thisincrease in the concentration of etchant species leads to an undesirablelowering of the oxide-to-mask or oxide-to-nonoxide selectivity.

However, removing the selectivity-controlling scavenging process fromthe primary process chamber and instead performing it in a separatescavenging chamber allows the selectivity to be manipulated independentof etch performance. Process parameters conducive to optimum selectivitycan be imposed in the scavenging chamber. Specifically, processparameters such as power input and temperature can be set to reduce theconcentration of available etchant species in the scavenged etchantgases to a level that results in a maximum reduction in the etch rate ofthe particular material desired to have an increased selectivity. Inaddition to the reduction of available etchant species, this optimizedscavenging process will introduce or “load” etch by-products into themodified etchant gas. These etch by-products are capable of inhibitingthe diffusion of similar by-products away from the surface of theworkpiece undergoing etching in the primary processing chamber. It isknown that the presence of etching by-products at the surface of aworkpiece will inhibit etching of the material associated with thoseby-products. Thus, selectivity is increased via this phenomenon as well.

Meanwhile, the process parameters tending to optimize etch performancecan be imposed in the primary processing chamber without having to alsoconsider the impact on selectivity. For the most part the advantageouschanges made to the etchant gases in the scavenging chamber, namely areduction of the available etchant species to desired levels and theintroduction of beneficial etch by-products, will not be undone in theplasma of the primary processing chamber. Thus, for example, the powerinput to the primary chamber can be set to maximize the etch rate of thematerial being etched from the workpiece (e.g. silicon dioxide) withoutconcern for also increasing the etch rate of materials not intended tobe etched from the workpiece (e.g. mask and non-oxide layers of theworkpiece). Granted, if the power level inside the primary chamber isextremely high, some of the etch by-products may be broken down toproduce free etchant species. However, if this is the case, it would bepossible to compensate for this release of etchant species by using thescavenging chamber to “over-scavenge”, thereby resulting in a net freeetchant species concentration in the primary chamber of the desiredlevel.

Another significant advantage of conducting the scavenging processoutside of the primary processing chamber is that there is no longer aneed to prevent deposition of etch by-products or etch resistantmaterials (such as the polymer created in a silicon dioxide etchprocess) on the chamber's interior surfaces. In the past, suchdepositions had to be avoided in order to keep the solid scavengingmaterial source form being covered, thereby becoming non-reactive withthe plasma. However, since the scavenging material source is nowresident in the scavenging chamber there is no need to completelyprevent the depositions on the surfaces of the primary process chamber.In fact, some amount of deposition on the chamber surfaces has abeneficial effect that could not heretofore been realized. Specifically,these deposits tend to trap “harmful” particles that could otherwisehave found their way to the surface of the workpiece where they coulddamage or destroy the devices being formed thereon. Granted, somecontrol of the amount of deposition may still have to be maintained. Forexample, in the case of the polymer formed in the plasma of an etchprocess designed to etch silicon dioxide from a silicon wafer, theremust not be so much deposition of the polymer on the chamber surfacesthat there is not enough left to adequately protect the mask andnon-oxide layers of the wafer. Regardless, it is much easier to simplycontrol the amount of deposits on the chamber surfaces, than it is tocompletely prevent them.

An inductive, capacitive or microwave discharge is preferably used tocreate the plasma in the scavenging chamber. FIG. 2A illustrates anembodiment of a scavenging chamber 200 configured to create a plasmausing an inductive discharge. The chamber 200 has disk-shaped lid 202and bottom 204, and a cylindrical side wall 206. An inductive coilantenna 208 is wound around the side wall of the chamber 200 andconnected to a radio frequency (RF) plasma source power generator 210through an impedance match circuit 212 to provide RF power into thechamber. Preferably, the side wall 206 is made of a dielectric orsemi-conductor material so as to not significantly inhibit the transferof RF energy into the chamber. The lid 202 and bottom 204 of the chambercan be made of any appropriate material, including a dielectric orsemi-conductor material, or even a metal such as aluminum or stainlesssteel. The above-described chamber structure is meant as an exampleonly. Many other chamber shapes are equally viable. For example, thescavenging chamber could have a domed-shaped top portion similar to theprimary processing chambers of many commercially available plasmareactors. The coil antenna in such a dome-shaped reactor could surroundall or a portion of the dome-shaped top portion. Essentially, anychamber configuration known to be employed for the primary processingchamber of a plasma reactor could also be used in constructing thescavenging chamber associated with the present invention.

Etchant gas is introduced into the scavenging chamber 200 via an inletline 214 that originates at a conventional etchant gas source (notshown). The etchant gas is modified inside the chamber as will bediscussed later and this modified gas exits the scavenging chamber 200via an outlet line 216. The outlet line 216 is connected to an inlet ofthe primary processing chamber, as described previously.

A hollow, cylindrical liner 218 is disposed within the chamber 200. Thisliner 218 is formed at least in part of a scavenging material andconstitutes the aforementioned scavenging material source. Preferably,the liner 218 is sized such that it slides into the chamber 200 with thelid 202 removed, and its exterior surface abuts against the interiorsurface of the chamber's side wall 206. This liner configuration allowfor easy installation and removal to facilitate its replacement once thescavenging material has been consumed. It is noted that a key advantageof the external scavenging chamber concept is that the consumable solidscavenging material source can be quickly and easily replaced withouthaving to open the primary processing chamber. Opening the primaryprocessing chamber is time-consuming and often quite a complexoperation. In addition, opening the primary processing chamber risks theintroduction of contaminants into the chamber and/or disturbing depositsthat have formed on the interior surfaces of the chamber. Contaminantsand loose deposit material can adversely affect processing conditionswithin the chamber, or could fall onto a workpiece being processed andcause damage to the devices being formed thereon. Thus, eliminating theneed to open the primary processing chamber to replace the scavengingsource is quite desirable.

The size of the scavenging chamber 200 is primarily dictated by the sizeof the liner 218, and the liner 218 can be of any size desired. However,there are some factors to be considered. First, the area of the interiorsurface of the liner 218 should be made large enough to ensure itsreaction with the plasma is sufficient to reduce the concentration offree etchant species to the desired level, given a particular RF powerlevel input to the coil antenna 208 and a particular flow rate of theetchant gas through the chamber 200. However, as discussed previously,some types of solid scavenging material sources (e.g. silicon) becomemore difficult to manufacture and more expensive as they become larger.Therefore, an opportunity exists to reduce the cost of the scavengingmaterial source by making it smaller. Increasing the RF power input orslowing the flow rate of the etchant gas could be explored as possibleways to achieve the desired scavenging effect while still employing arelatively small source. It is noted that smaller sources will requiremore frequent replacement. However, given the easy replacementcharacteristics of the proposed scavenging chamber 200, it is believedthe overall cost would still be less than employing larger sources.

It is also possible to modify the scavenging chamber 200 and liner 218to accommodate an optional temperature control apparatus 220 (shown indashed lines in FIG. 2A) used to control the temperature of the liner218. This optional temperature control apparatus could be anyappropriate type currently used for the same purpose in existing schemesfor controlling the temperature of a solid scavenging material sourcewithin the primary processing chamber of a plasma reactor. The additionof such a temperature control apparatus would provide yet another way tocontrol the scavenging process in the scavening chamber 200, albeit atan increased cost.

While the liner has been described as being cylindrical in shape, thisis done by way of an example only. The liner may take on other shapesdesired. In addition, the inner surface of the liner need not be smoothcylindrical wall and need not match the shape of the external surface ofthe liner. For example, the inner wall of the liner could advantageouslybe made up of a series of longitudinal ribs projecting into the hollowinterior of the liner and spaced periodically around the circumferenceof the wall. Such a configuration would increase the surface area of theinner surface of the liner over that of a simple cylindrical wall. Theincreased surface area would result in a greater interaction between theliner and the plasma formed therein, thereby increasing the scavengingeffect.

In the background section of the this patent specification an example ofa particular scavenging process was provided that involved the etchingof a silicon oxide layer to an underlying polysilicon conductor layerand/or to a silicon substrate of a semiconductor wafer usingfluorine-containing etchant species. The solid scavenging sourceemployed in this process was made of silicon or silicon-containingmaterials such as silicon carbide (SiC) or a silicon-based dielectric.However, other plasma-enhanced etch processes using different etchingchemistries can also benefit from the present invention. Generally, anyplasma etching process that employs etchant species scavenging tocontrol selectivity is a candidate. Generally, to increase selectivityto a particular material (i.e. lower the etch rate of the material incomparison to other materials present on a workpiece), this material isused to form the liner of the scavenging chamber. In operation, theconcentration of the particular etchant species in a plasma formed fromthe etchant gas will be reduced and the etchant gas will be “loaded”with the previously-discussed beneficial etching by-productsattributable to the reaction between the particular material and theetchant species. The combination of the reduced concentration of etchantspecies in the plasma and the presence of etching by-products results inan increase in the selectivity to the material used to form the liner ofthe scavenging chamber. Further, if it would be difficult or expensiveto form the liner of the particular material for which it is desired toincrease the etch selectivity, a related material that scavenges thesame etchant species and creates the same etch by-products (or similaretch by-products having the same inhibiting effect) can be used as asubstitute. The use of a substitute liner material would also provide adistinct advantage if it is known to more effectively scavenge theetchant species of interest or produce more of the desired etchinhibiting etch by-products. An example of etch processes andchemistries other than the previously-described silicon oxide etchscenario include a polysilicon and silicide (gate) etch processemploying chlorine (Cl₂) and/or hydrogen bromide (HBr) based etchantgases. This process require high selectivity to the overlying mask layerand a very thin, underlying gate oxide layer (e.g. 40-100 Angstoms ofSiO₂). Another example is a metal etch process where a high selectivityto the overlying mask is required. In both of these examples, excessconcentrations of etchant species in the plasma can detrimentally affectthe desired high selectivity.

In the case where the material for which it is desired to increaseselectivity is electrically conductive, the liner of the scavengingchamber will have to be modified to ensure sufficient RF energy reachesthe interior of the chamber to ignite and maintain a plasma. As shown inFIG. 2B, the scavenging chamber 200′ has been modified to include aslotted liner 218′. Specifically, the modified liner 218′ haslongitudinally-oriented rectangular slots 222 spaced periodically aroundthe liner's circumference, preferably at equal distances. The number andsize of the slots 222 is chosen to ensure a transfer of RF energy intothe interior of the chamber 200′ from the coil antenna 208 sufficient toboth ignite and maintain a plasma. It is noted, however, that althoughlongitudinally-oriented rectangular slots are shown, other slot shapescan be employed without deviating from the scope of the presentinvention. The only critical requirement is that sufficient RF energy betransmitted through the liner 218′ to the interior of the chamber 200′.

FIG. 3A illustrates an embodiment of a scavenging chamber 300 configuredto create a plasma using a capacitive discharge. The external parts ofthe chamber 300 are similar to those of the chamber of FIGS. 2A and 2Bin that there is a disk-shaped lid 302 and bottom 304, and a cylindricalside wall 306. Here again, etchant gas is introduced into the scavengingchamber 300 via an inlet line 308 that originates at a conventionaletchant gas source (not shown). The etchant gas once modified in thechamber 300 exits via an outlet line 310, which is connected to an inletof the primary processing chamber.

The scavenging chamber 300 differs from the previously-describedinductive discharge embodiment in that the inductive coil antenna isreplaced by a pair of electrodes 312, 314. In a first version of thecapacitive discharge embodiment shown in FIG. 3A, these electrodes 312,314 are resident within the chamber 300 and face each other fromopposite sides so as to form a plasma formation region therebetween. Oneor both of the electrodes 312, 314 can constitute the solid scavengingmaterial source of the chamber if at least one is made of the desiredscavenging material. The electrodes 312, 314 are also electricallyconnected to opposite sides of a plasma source power generator 316. Inthe case depicted in FIG. 3A, the first electrode 312 is connected tothe active side of the generator 316 through an impedance match circuit318 and the second electrode 314 is connected to the ground side of thegenerator. These connections can be reversed if desired. Since theelectrodes 312, 314 are resident within the chamber 300, the lid 302,bottom 304 and side wall 306 can be made of any appropriate materialincluding a dielectric or semi-conductor material, or a metal such asaluminum or stainless steel.

As with the previous embodiments, the electrodes 312, 314 are designedto slide easily into and out of the chamber 300 when the lid 302 isremoved, thereby facilitating replacement of the consumed scavengingmaterial source. This will preferably entail the use of an appropriatesecuring structure (not shown) within the chamber 300, as well as quickdisconnect-type electrical connections. As a vast number of differentsecuring arrangements can be employed and do not form a novel part ofthe present invention, no further details will be provided herein.

The size of the electrodes 312, 314 can be of any desired, however, theaforementioned design factors apply here as well. Specifically, the areaof the surfaces of the electrodes 312, 314 that react with the plasmashould be made large enough to ensure the concentration of free etchantspecies is reduced to the desired level, given a particular RF powerlevel input and a particular flow rate of the etchant gas through thechamber 300. However, as some types of solid scavenging materialsources, like one made from silicon, are more difficult to manufactureand more expensive as they become larger, it would be preferable to makethe electrodes 312, 314 relatively small. The key is to make theelectrodes as small as possible, but still able to provide the desiredscavenging effect at a reasonable RF power input level, a reasonableetchant gas flow rate and a reasonable replacement frequency.

One or both of the electrodes 312, 314 can also be modified like theliner of FIGS. 2A and 2B to accommodate a temperature control apparatus(not shown). Here again, this optional temperature control apparatuswould provide yet another way to control the scavenging process.

In an alternative version of the capacitive discharge embodiment shownin FIG. 3B, the electrodes 312′, 314′ are placed outside the side wall306 of the chamber 300′ on opposite sides thereof so as to face eachother and form a plasma formation region inside the chamber. In thisversion the electrodes 312′, 314′ can be made of any appropriateconductive material, and are connected to the plasma source powergenerator 316 in the same way as the previous version of this embodimentshown in FIG. 3A. However, because the electrodes 312′, 314′ are outsidethe side wall 306 of the chamber, the side wall is made of a dielectricor semi-conductor material so as to not significantly inhibit thetransfer of RF energy into the chamber. The lid 302 and bottom 304 ofthe chamber can be made of any appropriate material, including adielectric or semi-conductor material, or a metal such as aluminum orstainless steel. This version of the capacitive discharge embodiment isparticularly useful where the scavenging material is not conductive andso inappropriate for use as an electrode.

The solid scavenging material source in the version of the capacitivedischarge embodiment shown in FIG. 3B is preferably the same as thatemployed in the inductive discharge embodiment of FIGS. 2A and 2B,namely a hollow, cylindrical liner 320 formed of the desired scavengingmaterial. Additionally, the same design factors concerning size, ease ofreplacement and optional temperature control apply the liner 320, aswell.

The embodiment of FIG. 3A could also be advantageously modified tocreate a scavenging chamber configured to create a plasma using a directcurrent (DC) discharge. This can be accomplished by replacing the RFpower generator with a DC source (not shown) and making the electrodesfrom a conductive material. If the desired scavenging material is notconductive and cannot be used to form one or both of the electrodes,then the solid scavenging material source would preferably be theaforementioned hollow, cylindrical liner. Here again, thepreviously-described design factors concerning size, ease of replacementand optional temperature control of the scavenging material source applyto this DC discharge embodiment as well.

FIG. 4 illustrates an embodiment of a scavenging chamber 400 configuredto create a plasma using a microwave discharge. In this embodiment, thescavenging chamber 400 includes an applicator tube 402, preferably madeof a dielectric material such as sapphire. The applicator tube 402 isconnected at one end to an etchant gas source (not shown) via anappropriate inlet feed line 404, and at the other end to the inlet of aliner chamber 406. The liner chamber 406 is nearly identical to thechamber 200 of FIG. 2A and shares the similar design attributes andadvantages. This includes having a liner 408 that is identical to theliner 218 of FIG. 2A or any of its previously-described variations. Theliner 408 constitutes the solid scavenging material source of thescavenging chamber 400 of this embodiment. However, in this case theliner 408 can be made of any scavenging material including materialsthat are conductive because there is no need to couple RF power throughit. The outlet of the liner chamber 406 is attached to an appropriateoutlet feed line 410, which is in turn connected to an inlet of theprimary processing chamber of the plasma reactor. The applicator tube402 passes through a microwave waveguide 412 at a point in-between theinlet feed line 404 and the liner chamber 406. The microwave power iscoupled via the waveguide 412, thus creating a plasma inside theapplicator tube 402 from the etchant gas passing through on its way tothe primary processing chamber. A similar microwave discharge scheme wasdisclosed in a co-pending application, except that the referred toapplication included multiple applicator tubes whereas just one ispreferably employed in this microwave discharge embodiment of thepresent invention. The co-pending application is entitled DISTRIBUTEDMICROWAVE PLASMA REACTOR FOR SEMICONDUCTOR PROCESSING and has some ofthe same inventors as the present application, and is assigned to acommon assignee. This application was filed on Jun. 23, 1995 andassigned Ser. No. 08/494,297, now U. S. Pat. No. 5,702,530. Thedisclosure of the co-pending application is herein incorporated byreference.

The etchant species generated by the plasma within the applicator tube402 will exhibit a relatively high energy in comparison to such speciesgenerated within an inductive or capacitive discharge-created plasma.This high energy dictates placing the liner 408 downstream of themicrowave source 412. For example, if a microwave discharge-createdplasma were created within or adjacent to the liner 408 itself, theplasma energy would be so high that the etchant gas would beover-scavenged of etchant species. However, the etchant gas loses energyas it flows down the applicator tube 402, therefore the liner chamber406 is preferably located at a distance away from the microwave source412 that roughly coincides with an expected plasma energy range capableof creating the desired scavenging effect. This placement of the linerchamber 406 alone is not relied upon to control the preciseconcentration of etchant species scavenged from the etchant gas as itflows through the scavenging chamber 400. Rather, other control methodsare also employed. For example, as with all the embodiments of thepresent invention the power input to the plasma generating apparatus,which in this case is a microwave source, can be varied to control thescavenging effect. Additionally, the flow rate of etchant gas throughthe scavenging chamber 400 could be varied to assist in creating thedesired scavenging effect. The previously-described temperature controlapparatus can also be incorporated into the liner 408 to provide afurther control of the concentration of etchant species scavenged fromthe etchant gas.

While embodiments of the invention described thus far involve the use ofa single scavenging chamber, this need not be the case. For example, insome etching processes it is desirable to increase the selectivity tomore than one material wherein these materials require a differentetchant species to be scavenged and/or different etching by-products tobe loaded into the etchant gas. For example, the mask and someunderlying workpiece layer materials can fall into this category. Incases where multiple scavenging materials cannot be combined in a singlechamber without a detrimental effect on the scavenging process, two ormore scavenging chambers employing different scavenging materials can beincorporated into the plasma reactor. In this configuration, theindividual scavenging chambers are connected separately to the gasdistribution apparatus 510 of the primary processing chamber 512 of thereactor 500, such as the two chamber 502, 504 depicted in FIG. 5. Bothchambers could be connected to an etchant gas source 506, 508 containingthe same etchant gas and used to scavenge the same etchant species. Inthis way, each separate flow of etchant gas entering the respectivechambers 502, 504 would undergo the scavenging process. The scavengingmaterial used in the chambers 502, 504 would, however, be such thatdifferent etch by-products are loaded into the respective etchant gasflows. If desired, the multiple scavenging chambers could also beconnected to the same etchant gas source (not shown). Of course, thisconfiguration could also be used to scavenge different etchant speciesfrom the etchant gas and load the gas with different etch by-products.Further, the gas sources 506, 508 could contain different etchant gases,rather than the same gas depending on the scavenging requirements of theprocess being performed in the primary processing chamber of thereactor.

Any of the previously-described scavenging chamber embodiments can alsobe modified to solve another problem typical of etch processing in aplasma reactor. It has been explained that process parameters promotingoptimal etch conditions (such as a fast etch rate) within the primaryprocessing chamber are not necessarily conducive to maximizing thedesired selectivity. The same scenario also exist for other processfactors as well. For example, inert gases such as argon (Ar) and helium(He) are often excited in the plasma of the primary processing chamberof a plasma reactor for advantageous effect. However, the processparameters that promote optimal etch performance in the primary chamber,such as the RF power level, are not always conducive to effectivelyexcite these inert gas components of the processing gas. The problem canbe resolved by employing a separate excitation chamber external to theprimary processing chamber that excites gases at optimal conditions andfeeds the modified gases into the primary chamber. An appropriateexcitation chamber would be identical to any of the previously-describedscavenging chamber embodiments, with the exception that the solidscavenging material source is not needed and can be eliminated. To thisend, it is possible to modify an existing scavenging chamber to act asan excitation chamber by simply removing the scavenging material source.For example, in the embodiments using a liner as the scavenging source,it can be removed. In the capacitive discharge embodiment using internalelectrodes made of a scavenging material, the electrodes can be replacedwith ones made from a non-scavenging material. It is noted that as thescavenging liners and electrodes are designed for easy removal,converting a scavenging chamber to an excitation chamber without ascavenging source material can be readily accomplished. Further, if theionization process is not adversely affected by the presence of a solidscavenging material source, then these structure could remain in place,thus making the excitation chamber physically identical to thescavenging chamber.

An excitation chamber can be employed alone, or in conjunction withother ionization and/or scavenging chambers, as desired. In regards tothe later possibility, it can be readily imagined that a plasma reactoremploying multiple scavenging chambers (such as was describedpreviously) can be quite versatile. For example, if only one scavengingchamber is necessary, the other(s) can be idled. If only one excitationchamber is necessary, one of the scavenging chambers could be modifiedas needed, and again the other scavenging chambers would be idled. Inthe case where more than one scavenging process is required, thechambers can be employed as discussed previously. And finally, if morethan one ionization processes or simultaneous scavenging and ionizationprocesses are desired, the multiple scavenging chambers can beemployed/modified as appropriate to accommodate this need.

While the invention has been described in detail by specific referenceto preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention. For example, while the use of a solidscavenging material source is preferred and has been discussedthroughout the foregoing description, it is possible that a gascontaining the appropriate scavenging material could be introduced intothe scavenging chamber instead. This scavenging gas would react with theetchant gas in the plasma formed within the scavenging chamber. Theamount of gas introduced would be regulated to ensure the desiredscavenging effect is achieved. Thus, the term scavenging source as usedin connection with the present invention should be understood to includematerials comprising a gas, a solid, or even a liquid, unless indicatedotherwise.

What is claimed is:
 1. A method for scavenging etchant species from aplasma formed of etchant gas prior to the etchant gas entering a primaryprocessing chamber of a plasma reactor, the method comprising the stepsof: connecting a scavenging chamber at an outlet thereof to an etchantgas source and connecting the scavenging chamber at an inlet thereof tothe primary processing chamber; flowing etchant gas through thescavenging chamber from the etchant gas source to the primary processingchamber of the plasma reactor; irradiating the interior of thescavenging chamber to create a plasma from etchant gas flowingtherethrough; and disposing an etchant species scavenging source withinthe scavenging chamber to interact with the plasma to scavenge etchantspecies created by the dissociation of the etchant gas in the plasma andform etch byproducts comprised of substances from both the etchantspecies and the scavenging source.
 2. The method of claim 1, wherein thestep of irradiating the interior of the scavenging chamber to create aplasma therein comprises the step of employing one of (i) an inductivedischarge, (ii) a capacitive discharge, (iii) a direct current (DC)discharge or (iv) a microwave discharge.
 3. The method of claim 1,wherein the etchant species scavenging source comprises a solidscavenging material, and wherein the step of disposing the etchantspecies scavenging source within the scavenging chamber comprises thestep of making the source as small as possible while still maintainingits capability of scavenging sufficient etchant species so as to createa desired concentration of etchant species and etchant by-products in aplasma formed within the primary processing chamber.
 4. The method ofclaim 1, wherein the scavenging chamber comprises a removable lidcovering an access opening thereof and the etchant species scavengingsource comprises a solid scavenging material, and wherein the step ofdisposing an etchant species scavenging source within the scavengingchamber comprises sizing the source so as to facilitate its beinginstalled into or removed from the chamber through the access opening.5. The method of claim 1, wherein the etchant species scavenging sourcecomprises a solid scavenging material, the method further comprising thestep of controlling the temperature of the etchant species scavengingsource so as to assist in ensuring the source is capable of scavengingsufficient etchant species to create a desired concentration of etchantspecies and etchant by-products in a plasma formed within the primaryprocessing chamber.
 6. The method of claim 1, wherein the primaryprocessing chamber of the plasma reactor contains a workpiece undergoingplasma-assisted etch processing therein, said workpiece including atarget material, and wherein the step of disposing an etchant speciesscavenging source within the scavenging chamber comprises disposing asource comprising a scavenging material that scavenges the type ofetchant species from the plasma formed within the scavenging chamber andproduces the type of etch by-products that result in a lowering of theetch rate of the target material.
 7. The method of claim 1, wherein theetchant species scavenging source comprises a gaseous scavengingmaterial, and wherein the step of disposing the etchant speciesscavenging source within the scavenging chamber comprises a step ofintroducing into the scavenging chamber a sufficient quantity of thegaseous scavenging material to create a desired concentration of etchantspecies and etchant by-products in a plasma formed within the primaryprocessing chamber.
 8. The method of claim 1, further comprising thesteps of: connecting at least one additional scavenging chamber at anoutlet thereof to one of (i) the etchant gas source associated with thefirst scavenging chamber or (ii) a different etchant gas source, andconnecting each additional scavenging chamber at an inlet thereof to theprimary processing chamber; flowing etchant gas through each additionalscavenging chamber from its associated etchant gas source to the primaryprocessing chamber of the plasma reactor; irradiating the interior ofeach additional scavenging chamber to create a plasma from etchant gasflowing therethrough; and disposing an etchant species scavenging sourcewithin each additional scavenging chamber to interact with the plasmaformed therein to scavenge etchant species created by the dissociationof the etchant gas in the plasma and form etch by-products comprised ofsubstances from both the etchant species and the scavenging source.